Many types of input devices are presently available for performing operations in a computing system, such as buttons or keys, mice, trackballs, touch sensor panels, joysticks, touch screens and the like. Touch screens, in particular, are becoming increasingly popular because of their ease and versatility of operation as well as their declining price. Touch screens can include a touch sensor panel, which can be a clear panel with a touch-sensitive surface, and a display device that can be positioned behind the panel so that the touch-sensitive surface can substantially cover the viewable area of the display device. Touch screens can allow a user to perform various functions by touching the touch sensor panel using a finger, stylus or other object at a location dictated by a user interface (UI) being displayed by the display device. In general, touch screens can recognize a touch event and the position of the touch event on the touch sensor panel, and the computing system can then interpret the touch event in accordance with the display appearing at the time of the touch event, and thereafter can perform one or more actions based on the touch event.
Touch sensor panels can be formed from a matrix of row and column traces, with sensors or pixels present where the rows and columns cross over each other while being separated by a dielectric material. Each row can be driven by a stimulation signal, and touch locations can be identified because the charge injected into the columns due to the stimulation signal is proportional to the amount of touch. However, the high voltage that can be required for the stimulation signal can force the sensor panel circuitry to be larger in size, and separated into two or more discrete chips. In addition, touch screens formed from capacitance-based touch sensor panels and display devices such as liquid crystal displays (LCDs) can suffer from noise problems because the voltage switching required to operate an LCD can capacitively couple onto the columns of the touch sensor panel and cause inaccurate measurements of touch. Furthermore, alternating current (AC) adapters used to power or charge the system can also couple noise into the touchscreen. Other sources of noise can include switching power supplies in the system, backlight inverters, and light emitting diode (LED) pulse drivers. Each of these noise sources has a unique frequency and amplitude of interference that can change with respect to time.